


Beaten Black And Blue

by MeGaLoTrash



Series: Multiverse One shots [5]
Category: Undertale (Video Game)
Genre: Alternate Universe - Swapfell, Alternate Universe - Underswap, BBQTacos - Freeform, BDSM Scene, Boot Worship, Double Penetration, Ecto-Penis (Undertale), Ecto-Vagina (Undertale), Ectobiology, Explicit Language, Explicit Sexual Content, Humiliation, M/M, Masochism, Multi, Pet Play, Poisonberry - Freeform, Puppyberry, Spanking, Threesome - M/M/M, rottenberry, swapfellcest - Freeform
Language: English
Status: Completed
Published: 2017-08-24
Updated: 2017-08-24
Packaged: 2018-12-19 12:50:46
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,511
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11898105
Author URL: https://archiveofourown.org/users/MeGaLoTrash/pseuds/MeGaLoTrash
Summary: Slim was a monster who never wanted for muchBut if he ever did find something he wanted had a tendency to pursue it obsessively.





	Beaten Black And Blue

**Author's Note:**

> Written with the lovely @Damnedxfate in mind, thank you for letting me run with the pairing idea <3

Slim was a monster who never wanted for much. Considering his chaotic home, it was a bit of a marvel that he was so content. But he had his lord, a roof over his head and a reasonable supply of his vices, everything else was just background noise. The problem was that if he ever found anything else he wanted, he had a tendency to pursue it obsessively. 

Which is exactly what happened when he laid eyes on a little blue skeleton.

It had been a bit of a cluster fuck when his universe collided with a few others, and it had be partially his fault for toying around with the machine in the shed. He and a few of his alternates had figured out what was going on pretty quickly and had rushed to stop their brothers from ripping each other apart. That was when he saw him, he was prying his lord of a sweet version of himself when he saw a flash of Blue and a defiant expression. He had almost forgotten what he was doing before Black clipped him around the skull and demanded explanations.

While all the skeletons were discussing what happened he acted the part of the loyal dog at his brother's side. He couldn’t help steal a handful of looks though; first at his lord, then at the other skeleton, then back at his lord. Apart from a few minor aesthetic differences and the lack of scars they were nearly identical. Slim loved his lord so much, was completely infatuated with him even after years together, and this new skeleton was just like another version of him. 

This skeleton was also a Sans but the group had decided to call him Blue for confusion's sake, and they called his brother Black even when he protested. _black and blue, blue and black_ , even their nicknames sounded the same and it was driving him crazy. Slim knew better than to speak out of line and approach them right away though. Being one of the more observant ones in the group he noticed how certain pairs or groups were drawn to each other. He knew it wouldn’t be long until someone cracked and put some kind of proposal forward, he just had to be patient.

He was surprised it was the lazy comedian who brought it up, he was impressed and he could tell his lord was as well, they may have underestimated him. He knew Black would agree right away to their arrangement, they had talked about it right away and set limits and boundaries within their dynamic, it was second nature. Slim couldn’t help but shiver when he heard Blue had agreed, but he could tell the little skeleton was already enamoured with the red edgelord, who ended up getting to him first. 

Slim didn’t care. He was more than happy to play along with their little groups ‘arrangements’ if it meant he was within a chance of getting his hands on that little prince. Just thinking about having him in his grasp made him whine internally. He wanted to know just how similar he was to his lord, and what their differences were. He wanted to know if their cocks would taste the same, whether Blue was innocent as he acted or if he would share his lords commanding streak. His fantasies kept switching between the two of them until he couldn’t take it any more and it started to affect him externally. 

Black had known something was different with him straight away, he waited until they were alone in their room to bring it up. There was no hiding anything from his lord. Slim had scrambled a little, not knowing exactly how to word what he wanted in a way that his lord would understand. It wasn’t that he wanted to fuck anyone else, It was a _need_ to be with his lord’s copy. He needed it so much he could hardly get his words out, Black gave him a quick swat of his riding crop against his leg to get him to focus.

“Out With It Whelp-” Black moved closer to where he was kneeling on their bedroom floor, eyes still piercing even in the darkness. “-Or I’ll Have To Beat It Out Of You, Though You’d Probably Love That Too.”

While having Black hurt him until he caved did sound tempting, Slim knew this might be the one chance he had to say what he wanted. He also didn’t want to lie, He and Black told each other everything. He knew Black well enough to tell the way he was shuffling in his stance that not knowing was making him a little nervous. Slim had to put his own fears aside, not saying anything was worse than upsetting his lord by lying.

“I have been having lustful thoughts, involving o-one of your alternates” Slim confessed, waiting for Black’s response before he elaborated.

Even in the dark of their bedroom he could see Black pretty clearly as he leant down towards him. Black lifted up his chin so they were making eye contact, something he struggled with at the best of times but he tried hard to resist the urge to look away. There wasn’t any actual anger in his face as he expected, Black had a stern look about him as he always did, but his smirk was a give away to the fact he was enjoying playing with his pet.

“It’s Blue Isn’t It?” Black’s grin got wider when Slim nodded in response, he looked almost manic as he placed a heeled boot against Slim’s crotch “I Knew You Wouldn’t Be Able To Resist, He Looks Very Familiar Doesn’t He?”

All Slim could do was nod again in response, Black’s grip around his jaw was tight and he felt powerless under the sting of the boot against his sensitive ecto-flesh. Rather than take offence, Black seemed to be revelling in how much Slim wanted Blue. His lord wasn’t a fool, even though he didn’t approve completely over how soft some of the alternate universes were, he couldn’t deny the similarities between them. 

“So Tell Me More About Your Thoughts... And I Want Specifics” Black unzipped his shorts as he spoke, making a move to free his cock from his pants.

“i-i’ve fantasised about kissing him- about what his cock would look like- w-whether he would taste like you as w-well m’lord” Stretch confessed.

Blacks hand was still around his jaw even if his grip had lessened, keeping him still as he used his other hand to work over his cock close to Slim’s face. Black was teasing him, he knew how much Slim wanted to please him but he was holding himself just out of reach. _fuck this is hot_.

“Keep Going” Black’s hand was working faster on his cock, clearly enjoying having control over him. 

Slim could tell his words were having an quite effect on his brother. He revelled in being able to do that to him, and decided to try and push him over the edge. 

“i’ve thought about you fucking me while blue’s watching. i want him to join in. i want him to fuck me and have you tell him what to do. i really want both of you at once-” slim realised he was whining now as he fruitlessly moved his hips in search of the friction Black wouldn’t grant him “- please m’lord i can’t stop thinking about it”

Black didn’t answer him, he was already too far gone. Slim felt his jaw being nudged open before Black hissed through his teeth and covered his face in cum. Slim whined, he wanted nothing more than to lick up what he’d been given, but he didn’t had permission yet. His bones felt on fire and the magic in his pelvis was embarrassingly wet, he could probably cum himself with a few light touches at this point. It was driving him insane,but he loved being tortured at his lord’s whim. It just made the eventual release so much more intense. 

“Mhmmm. Good Boy, Make Sure To Clean Yourself Up” Black slowed his hand before pulling his hand off Slim’s jaw and zipping his shorts back up. 

“but m’lord i haven’t-” 

“You’re Not Going To Cum Mutt. Consider It Your Punishment For Not Telling Me Sooner-” Black leaned down to bring their faces level again, darting his tongue out and give Slim’s teeth a teasing lick “-Plus, If You Want Anything Like What You Mentioned, Wouldn’t It Be Better To Hold Out Until I Arrange it?”

Black turned on his heels and left him there, a panting desperate mess. It took him a moment to process what his brother had said through his clouded thought. _Until I Arrange It_. Slim couldn’t believe his luck, not only was his lord not really mad at him, he was actually considering his request. The possibility of having Blue within his grasp was a very real one, and if Black’s reaction was anything to go by, his lord was probably going to be involved too. Slim drew his long tongue over his face, savouring the taste and pushing down his own arousal. It was going to be hard to wait, but it was going to be worth it. 

 

The first few days were torture, Black had decided to put him on a complete sexual ban until he said otherwise. He had some alone time over the week, but he knew Black would be able if he’d managed to get off. He couldn’t help hug his pillow a little tighter at night as the week dragged on. His dreams were filled with visions of two figures looming above him, both demanding his attention. He woke up more than once practically fucking his pillow, arousal dripping between his legs.

 

He trusted his lord’s methods, but if something didn’t happen soon... 

Slim couldn’t help get excited one evening when he was instructed to leave their home for a few hours and not return until instructed. Black would not give him any reason as to why, he neither denied nor confirmed whether he’d been successful in speaking to Blue. Slim didn’t press him for an answer, more than happy to do as he was told. Whatever his lord had planned would be worthwhile, it always was. He had ventured to Muffet’s bar, but he couldn’t help check his phone every five minutes in anticipation.

When he finally got the message, he was out the door in seconds before even saying goodbye. He was careful not to let his guard down as he found a quiet place to take a shortcut home. Even though he wasn’t thinking completely clearly, he didn’t want to attract any unwanted attention. He appeared in the living room and quickly rid himself of his outdoor clothing before heading upstairs. He could feel the erratic thump of his soul in his chest as he knocked on their bedroom door softly. 

 

“Come In”

 

Slim walked into the darkened room, quickly shutting the door behind him. He knew what was expected of him. He walked a few paces forward and dropping to his knees on a leather mat that they usually started on. He wanted to look up and find out just what was going on, but he knew the rules his lord had set and wasn’t going to break them. 

Shiny black boots walked into his line of vision and he caught sight of the familiar riding crop. He felt his soul sink a little at there only being one pair of legs before him, but he ducked lower regardless as a foot was lifted up towards him. He pressed his tongue flat against the cool leather, lavishing it with attention. He knew how much his lord enjoyed feeling of being worshipped from head to toe, and he enjoyed giving it too him. It seemed to be having more of an effect on him that usual, Slim could hear little whimpers coming from his brother which spurred him on further. 

“Told You He’s Well Trained”

The voice that was unmistakeably his lord came from behind him. Slim froze mid-lick in confusion before slowly raising his eyes up to the skeleton he was pleasing. He looked like Black. He was wearing cut off leather shorts, way too many straps, and a purple scarf; all from Black’s wardrobe. The only thing that gave the game away was bright blue eyes in place of purple and scars. _oh fuccckkk_. He had been unknowingly servicing the monster of his dreams, and seeing him all dressed up _exactly_ like his lord. He same flicker of a smirk on his face before Blue pushed back on his chest with his boot, making him lose balance and fall over. 

“I Don’t Know Black, Seems Like He Needs A Few More Lessons In Control, Don’t You Think?” Blue mimicked Black’s tone as he spoke.

Slim was finding hard to stay in touch with reality, he was so overwhelmed. Black had moved from behind him and now stood next to Blue sneering down at him. All his fantasies were coming true and then some, he should have known his lord would of been able to come up with something even better than his wildest dreams. He could tell both skeletons were enjoying his shocked and hungry gaze. Black had even slipped a hand around Blue taking advantage of the more revealing outfit.

“You’re Right. Mutt, Stay. Don’t Move Until I Tell You I’m Going To Have Some Fun With Our Guest” Black Purred, leaning into Blue further.

Slim could feel drool starting to drip down his jaw watching the scene before him. Black moved to stand behind his double, attaching his mouth to Blue’s cervical vertebrae. Slim saw Blue’s eyes lights fill with stars as he arched into his doubles touch. Black chuckled darkly against him before biting down lightly, causing Blue to let out his first moan. Slim didn’t know where to look, everything about the scene before him was such a turn on. In a way he was glad he was just watching for now, he wanted to permanently store the scene in his memory. He tried to ignore the sweat trickling down his skull as his bones felt on fire with arousal. 

Black moved over to the bed, moving Blue along with him and into his lap. Black was working hands up under his top to tease across sensitive bone, revealing in the fact that Blue seemed to share his sweet spots. From Slim’s position he was able to see clearly what was going on but the low light made it harder to tell them apart. Black continued his exploration under Blue’s shirt and used his free hand to tilt his jaw back and capture him in a kiss. Blue seemed worked up already, pressing his tongue towards Black straight away and moaning in his mouth. In the dark Slim could make out their magic meeting and meshing together. He whimpered softly wanting nothing more than to be able to join them.

Both of the skeletons on the bed were noticing the effects their show was having on Slim. Even though they were wrapped up in each other they kept stealing glances at where he was knelt, which just made him feel even hotter. He knew it was part of their game to tease him, but knowing they both wanted him almost made him cave and walk over to the bed. Black probably sensed his impatience so he turned up the volume, moving the hand from under Blue’s top to palm the front of his shorts. 

Blue reacted instantly, first by arching up into Black’s palm before grinding his hips back against Black’s own erection. Slim’s soul fluttered upon seeing his lord caught a little of guard, his stoic mask slipping for a second before he doubled his efforts and grouped Blue’s length harder. Slim heard the familiar sound of a zipper before he was blessed with his first sight of Blue’s cock. He could feel drool dripping down his maw freely now as his mind raced to keep up. Blue’s cock, bar the colour change, looked _exactly_ like his lords and growled at the thought of having both of them just a few feet away.

“You’ve Been A Good Boy So Far. Crawl” Black commanded, shifting Blue off his lap to pull down his own zipper. 

Slim didn’t need telling twice, he bowed his head and crawled the few feet to the bed. He was a little embarrassed indulging in this type of humiliation in front of Blue, but he trusted his lord to have thoroughly explained his preferences if he was asking him with Blue here. The idea of Blue knowing what he liked, _and agreeing to take part_ made him crawl just a little bit faster. He wanted so badly to give himself over to pleasing them, but he appreciated that left to his own devices he’d probably be overly enthusiastic. He trusted Black to know best, he probably had the whole evening planned out.

“You’ve Redeemed Yourself-” Blue’s voice was surprisingly steady as he brushed phalanges across Slim’s skull “-And Good Puppies Get Rewards”

Slim felt himself shiver at Blue’s commanding voice, he was slipping into the role so well he couldn’t help be impressed. But the variation of his usual nickname reminded him that Blue still had differences from his lord, he couldn’t wait to find them all out. Blue made quick eye contact with his copy who gave him an enthusiastic nod and a big smirk. Slim didn’t have to wonder for long what that exchange meant as he felt the hand tighten on his jaw and his mouth directed towards Blue’s crotch. 

Blue didn’t waste time in thrusting his cock into Slim’s eager mouth. Slim felt his whole frame shake at finally getting his hands on the object of his desire. Blue’s pace was rough and fast, just like he liked, and he felt his eye sockets begin to tear up from the sensation. He could feel both of them watching where he and Blue were joined, he decided to give them more of a show. Pulling back against Blue’s grip slightly he wrapped his tongue around the tip of his cock whilst looking up at his new lover. Blue groaned at the new feeling, losing himself for a moment, Slim felt his soul soar.

“That’s A Dirty Tactic Mutt, Come Here” Black growled before jerking Slim’s skull in his direction and replacing the cock in his mouth with his own.

Slim moaned around Black’s cock, eagerly taking the familiar length in deep. Without prompting he raised a hand to wrap around Blue’s erection, earning moans from both the skeletons and prompting them to move closer together. Slim kept up the pace on them both, occasionally switching when permitted to keep them slick. Slim was in heaven with how amazing he felt, even if his pants were practically drenched with his own arousal. For a moment he thought his lovers may want to finish with him until he was roughly dragged to a standing position before being thrown on the bed.

“Pants Off, Now” 

Slim wasn’t entirely sure who the command came from, but he obliged any ways. Usually he would make a show of stripping but currently he just needed to be touched, he didn’t even bother to be coy about it, eagerly spreading his legs. 

“Eagar Mutt? Sorry You’ll Have To Wait, I Have A Lesson To Teach” Black instructed, coming closer to the bed and caressing his leg teasingly “Turn over. All Fours, And Stay Quiet”

Slim flapped his mouth a few times like a fish out of water as he processed what Black wanted. This was a frequent part of their usual sessions, but had no idea that Blue would be interested this type of play. The little prince was just full of surprises. He turned over and got into position, fully aware that the ass he was able to make with his magic was visually appealing. He waited, stilling his shaking hands as he mentally prepared himself for whatever they had planned. 

“It’s Always Best To Start With Your Hand To Gauge Their Tolerance, You Can Start Over Clothing But-” Black spoke calmly, which meant the hard swat to his sensitive flesh made him flinch “-With Ones As Experienced As This, You Don’t Need As Much Warm Up”

Black continued to give him hits with his hand, gradually increasing the frequency and power behind the strikes. Slim couldn’t help let out a few moans even when he’d been told to keep quiet. Finally being touched after being denied for so long was amazing, but the sting of pain kept him a little more level headed. He understood now why Black wanted to introduce this aspect, nothing but pleasure would have him floating away far too quickly.

“Blue, Grab The Crops Again. Usually We Work Up To Something Harder, But Since It’s Your First Lesson We’ll Go Easy On Him” Black Instructed.

Slim could hear rustling behind him before he felt the soft leather of one of his lords crops press against his backside. Black went on to instruct and demonstrate how to use the crop correctly, but their voices faded into white noise for him. He felt utterly exposed in the position, and being used as a tool for them both just hit a desire deep inside him. Slim couldn’t keep up with where the next hit was going to come from, or who was dealing it out. He felt objectified and used, and he loved it. He knew he could stop them at any point with his safe word, but he was enjoying the feeling of being totally at their mercy. Slim hadn’t even noticed how loud he was moaning until Black stop abruptly and squeezed his beaten flesh.

“Seems We’ll Have to Wait For Future Lessons, This Cunt Is Begging To Be Fucked-” Black groaned for effect when sinking a digit into Slim’s entrance, making him cry out more. “-So Wet Mutt, I Think You’re Ready For What We Have Planned”

Blue sat next to Slim on the bed, drawing him into a quick kiss while Black continued to work his fingers in and out teasingly. After a few minutes Black help to manoeuvre them both to the edge of the bed, with Blue sat up and Slim straddling his lap and facing away. Any questions over their position died on his tongue when he realised he was inches away from impaling himself on Blue’s erection. He was more than ready but he waited for the go ahead from the other skeletons before sinking down with a moan. 

“W-Wowsers Slim, You Feel Really Good” Blue groaned against him. 

Slim could feel Blue’s cock pulsing inside him as he started to rise up and down in his lap. Blue had one hand braced against the bed but the other was working up under his forgotten tank as he panted heavily against him. Black Stood before them both, half heartedly stroking himself while taking the delicious scene before him. Slim tried to focus and figure out what his lord was planning but it was _really fucking hard_ with Blue meeting his thrusts in a brutal pace. He could feel the sting of his abused backside hitting Blue on every thrust, which mixed with hitting his sensitive spots was enough to make him cum embarrassingly early with a drawn out moan. He didn’t even have much of a warning before he felt himself clamping down around Blue’s cock and covering him with his release. 

Blue only slowed his pace slightly, not fully stopping as he rode him through his orgasm. Slim looked to Black to see if he was in for a punishment for cumming without permission but Black only leered at him lecherously. _he planned that too_. Black was closer to them now, leaning into stretch before grazing a digit lightly over his folds. It was when Black inserted a finger in alongside Blue’s still pumping cock that the penny dropped. The position they were in, working him up, and wanting him to cum already all made sense. Black wanted to fuck him at the same time as Blue. Slim wouldn’t have been able stop moaning even if he wanted too.

“Figured It Out Huh? Think You’re Ready Mutt?-” Black was lining himself up while asking, already knowing the answer but waiting for his nod just incase.”-Good boy”

Blue stilled inside him while waiting for Black to push in alongside him, but none of the trio could keep quiet when Black managed to get most of the way in. Slim had some experience in using multiple toys like this, but nothing compared to being sandwiched between two writhing bodies. It wasn’t the most coordinated movements as they moved together frantically, but it didn’t matter. He was already close again and no matter how both his lovers tried to hide it, he could tell they were two. Black leaned into him further, bringing their tongues together in a messy kiss before starting to thrust in harder. 

He wasn’t sure who came first. He could feel himself tightening around the same time he heard Blue cry out his name, his _real_ name. It was enough to topple him over as he felt Blue start to fill him up. Even through his pleasure haze he didn’t miss Black’s peak. His lord wasn’t always a vocal lover, but when it was particularly intense for him he’d let out a drawn out moan before his eye lights briefly turned to hearts. Black finishing made him slump more of his weight on top of them both as they laid in a panting, twitching heap. 

 

“That Was Amazing-” Blue said when Slim finally rolled off him to catch his breathe. Blue leaned over to place a kiss against his skull before jumping up off the bed “-I’m Go Grab A Shower Before Bed, See You In A Few Minutes”

Blue was out the door grabbing a bag on the way, giving the brothers a small moment of privacy. Black sat down next to Slim on the bed and embraced him hard for a moment, muttering a few praises about how well he’d done. Slim was going to ask what Blue meant, but Black had read his expression and started filling in the blanks. 

“One Of Blue’s Conditions Before We Did Anything Was Him Staying The Night, And Even I Have To Admit Having You Both In My Bed Tonight Sounds Wonderful” Black smiled down at him before placing a chaste kiss on his teeth “-But We’re Showering Too, You Look More Filthy Than Usual Mutt. It’s Very Distracting”

Black pulled him too Slim feet before leading him by the hand to join Blue in the Bathroom. Just seeing the way Black’s face lit up when they entered the bathroom made him realise they weren’t going to be getting clean any time soon.

**Author's Note:**

> Hope you enjoyed (some of this may have been kinktober practice btw)  
> Comments and feedback always welcome, here or @MeGaLoTrash


End file.
